1. Field of the Invention
The invention relates to the design configuration of so-called calender rolls. These are rolls employed preferably in the glazing or smoothing section of paper machines, i.e. at the position at which the the already completed paper web receives its surface finish. However, corresponding rolls may also be employed, for example, in rolling mills for plastics films or in coating webs of fabric or paper.
2. Description of the Prior Art
Current prior art calender rolls are generally heated, various methods for heating the roll being applicable. The invention relates to rolls through which the heated heat transfer fluid, such as, for example, water or oil passes. Heating the roll in this case results by heat transfer from the fluid to the body of the roll. Likewise the considerations in accordance with the invention are applicable for rolls which are correspondingly cooled, i.e. by means of fluid passing through, the temperature of which is below that of the body of the roll.
One version of such rolls having a fluid through-flow is the so-called peripherally drilled roll. In this the body of the roll comprises axial parallel drilled passages distributed evenly on the circumference and as close as possible to the surface, through which the fluid flows. To achieve a consistent heating (or cooling) of the body the fluid is directed from one end of the roll to the other and back to the discharge end. In the so-called DUOPASS roll a return (=back flow) drilled passage is assigned to each supply (=inflow) drilled passage, two of the supply passages each corresponding to one return drilled passage in the so-called TRIPASS 2 roll.
To produce the connection between drilled passages for the supply and return flow diversion passages are arranged in the corresponding end of the roll. The end of the roll is generally provided with a bolted trunnion in which the diversion passages are located. However, in rolls having so-called floating bearings the diversion passages may be located between the shell of the roll and the mounting axle. Such rolls are termed to have xe2x80x9cfloatingxe2x80x9d bearings when a roll shell open at the ends is hydraulically supported by a fixed central axle about which it rotates.
It is particularly the case with large diameter calender rolls that the roll features in addition to the described peripheral drilled passages an enlarged centerbore serving to save mass and weight and which is maintained free of the heat transfer fluid. In this way the weight of the roll remains low and the volume of heat transfer fluid required is reduced, simultaneously resulting in savings in energy costs.
Practically all of the rolls of this type, when having a large diameter, are driven by means engaging a trunnion, i.e. are entrained not only by the material web passing through or by the counter roll. In this case the fluid is supplied through the trunnion facing away from the drive end, the returning fluid passing through this trunnion and then also leaving the roll. In the trunnion at the drive end the diversion of the fluid from the supply to the return drill passages is provided.
To limit the quantity of heat flowing into the diversion trunnion both the supply and outlet passages and the peripheral drilled passages in the immediate vicinity of the trunnion and the diversion passages in the trunnion are insulated. Without this insulation the trunnion would be heated up stronger than the body of the roll and expand radially more than the body of the roll. Due to the existing bolted connection this expansion would be transferred also to the body of the roll to aggravate the so-called Oxbow Effect occurring in any case at the edges of the body of the roll. Since to avoid being damaged at the edges the material web guided over the body of the roll needs to be slightly narrower than the so-called web length of the roll, the loss in heat of the roll body at the narrow portions of the web remaining free at the end of the roll is less than otherwise at the roll surface, as a result of which the ends of the roll when heated feature a larger diameter than the roll portion located further towards the middle of the roll. This effect is highly undesirable for treatment of the material web. Attempts are made to obviate this effect, more particularly, by arranging for a reduced heat supply to the end of the roll or to provide special cooling thereof. Any amplification in this effect as may materialize from an additional increase in temperature of the trunnion is unacceptable.
The non-driven trunnion is as regards amplification of the Oxbow Effect particularly critical since the supply and discharge passages are also located therein. On the other hand at the drive end of the roll an overcompensation of the Oxbow Effect may occur under certain operating conditions. Since at this end only diversion passages exist for the heat transfer fluid this trunnion receives significantly less heat energy than the opposite end. In addition, the bearing locations of the driving trunnion need to be maintained cool. As a result of this heat energy flows away from the end portion of the roll to the drive end via the trunnion to the bearing locations. The trunnion itself remains significantly cooler and is expanded corresponding less than the body of the roll. The same negative effect materializes from the usually solid, central portion of the trunnion extending into the centerbore of the body of the roll and thus obstructing the expansion of the trunnion flange corresponding to the edge of the roll body to which it is bolted. The roll edge of the drive end of the roll thus becomes smaller in diameter then otherwise for the roll web. The web of material in this portion is exposed to less pressure than the remaining web length of the roll which in paper production results in so-called hard edges of the paper web or even under extreme conditions in rupturing of or bursting open the paper edge. Before being reeled such edges need to be cut off. The resulting loss in production web may amount to serious percentages of production as a whole.
The purpose of the present invention is to counteract the problems described at the drive end of the heated calender rolls without making the manufacture of the drive end trunnion appreciably more expensive. For this purpose it is provided for that bypass passages are arranged in the drive end trunnion, through which part of the heat transfer fluid is returned from the peripheral supply drilled passages through the center of the trunnion back into the return drill passages. In addition, the center of the trunnion may receive a centerbore sealed off from the centerbore of the roll in which heat transfer fluid can collect to boost the increase in temperature. At the same time, this drilled passage improves the ability of the trunnion to expand so that it presents less resistance to the expansion of the body of the roll.
In a further aspect in accordance with the invention control means are arranged in the bypass passages in the drive end trunnion permitting external adjustment, through which the flow of heat transfer fluid and thus the increase in temperature of the trunnion can be influenced in keeping with the given operating conditions. This enables an optimum control of fluid flow when starting the calender.